The present invention relates to a CDMA (Code Division Multiple Access) mobile communication system and, more particularly, to a hand-off method and apparatus used at the boundary between frequency layers in a CDMA digital cellular system having a plurality of frequency layers.
In a CDMA digital cellular system based on the IS-95 communication means defined by TIA (Telecommunications Industry Association), a plurality of frequency layers are prepared in accordance with traffic. For example, a large number of frequency layers are prepared in a large-traffic area such as an urban area, whereas a small number of frequency layers are prepared in a small-traffic area such as a rural area.
According to the-hand-off scheme used in a conventional CDMA digital cellular system having a plurality of frequency layers, an adjacent cell at each frequency layer boundary indicating the boundary between frequencies has a device for transmitting only a pilot signal having the same frequency as the frequency layer of a self-cell so as to cause a mobile unit to send a notification indicating a destination cell, thereby performing switching (hand-off) to the frequency prepared in the adjacent cell. Alternatively, an adjacent cell has a device capable of measuring the strength of an electric field from a mobile unit using a frequency which is not assigned to the adjacent cell. With this arrangement, the system recognizes a destination cell from the measurement result, and performs hand-off to the frequency prepared in the adjacent cell.
The following problems are, however, posed in the above conventional scheme.
First, if a given frequency used in a given cell is not prepared in an adjacent cell, a device for transmitting only a pilot signal having the given frequency must be installed. Alternatively, a device for measuring the strength of an electric field from a mobile unit using a frequency which is not prepared in the self-cell must be installed. In any case, therefore, the equipment investment increases.
Second, it takes-much time to execute hand-off, so that a poor-speech-quality period is prolonged. This is because, the base station measures the strength of an electric field from a mobile unit upon reception of a notification indicating a deterioration in speech from the mobile unit. In addition, the possibility that the mobile unit will move out of the service area before the execution of hand-off increases.
Third, since a destination cell is determined after the strength of electric fields from all adjacent cells are measured, the time interval between the instant at which the mobile unit notifies the base station of a deterioration in speech quality and the instant at which hand-off is executed is longer than that in other schemes. For this reason, the second problem described above become more noticeable.